1. Field of the Invention
The present invention relates to a process for the preparation of purified xanthan gum. More particularly, it is directed to a process for the improvement of the transparency of a fermentation broth containing dissolved xanthan gum as well as aqueous solutions of xanthan gum by enzymatic treatment of the broth or aqueous solution.
2. Description of the Related Art
Xanthan gum can be obtained by conventional fermentation of a xanthan gum-producing bacterium, e.g., Xanthomonas campestris, a species in the genus Xanthomonas, and the like. It is recovered by precipitation with isopropanol from a fermented broth (This compound and process for preparing it are described in U.S. Pat. No. 3,659,026, column 4.)
Other processes for the manufacture of xanthan gum use, in place of Xanthomonas campestris, such other known xanthan gum-producing bacteria as Xanthomonas carotate, incanae, begoniae, papavericola, translucens, vasculorum, and hederae. All these species can produce a xanthan gum fermented broth.
U.S. Pat. Nos. 5,310,677, 5,194,386 and U.S. patent application Ser. No. 07/826,095, filed Jan. 27, 1992, the contents of each of which are incorporated herein by reference, discloses several hyperproducing strains of X. campestris and methods for obtaining these strains. In particular, two strains identified as X50 (ATCC 55429) and X59 (ATCC 55298), each of which is available from the American Type Culture Collection pursuant to Budapest Treaty, have been deposited on Feb. 18, 1992, with the American Type Culture Collection in Rockville, Md., and all restrictions of the availability to the public of the materials so deposited will be irrevocably removed upon the granting of a patent thereon. X50 and X59 are obtained by a selection procedure described in the above U.S. Patents, starting from publicly available strain NRRL 1459 which is referred to as X55. The broth at the end of fermentation of such strains contains from 0.5 to 2 percent by weight of undissolved matter including unconsumed nutrients and bacterial cell residues, in addition to about 2 to 5 percent by weight of xanthan gum. The aqueous solution of xanthan gum separated by extraction from the broth has low transparency. This presents problems in the fields where clear products are required, such as, the food and cosmetic industries, and in applications for secondary recovery of petroleum.
Known methods for the purification and clarification of xanthan gum, and solutions and fermentation broths thereof depend on centrifugation or cake filtration for the removal of the undissolved matter from the broth. In either case, however, the broth as obtained from the fermentation is so viscous that it must be diluted with water which adds to the cost and complexity of the process and makes such methods impractical. A variation of those methods suggests heating of the broth to enhance its filtration properties.
One effective approach is solubilizing the undissolved matter in the broth by enzymatic treatment. Many proposals have hitherto been made in this direction.
For example, (Japanese Patent Provisional Publication (Kokai) No. 50-121493) and U.S. Pat. Nos. 3,966,618/1976, 4,010,071/1977, and 4,165,257/1979 propose clarification by use of alkaline protease and neutral protease. They report, in fact, that xanthan gum solutions cannot necessarily be made absolutely clear or transparent and the solutions retain some degree of turbidity.
European Patent Publication No. EP 78621, British Patent Application No. 8,132,564/1981 (Japanese Patent Kokai No. 58-81792) teaches the use of acidic protease and neutral protease for this purpose. U.S. Pat. No. 4,119,491 (Japanese Patent Kokoku No. 62-44918) teaches bringing a solution that results from protease treatment into contact with a siliceous solid matter and then removing the cellular debris from an aqueous polymer solution. U.S. Pat. No. 4,431,734 (Japanese Patent Kokai No. 57-202303) proposes an enzymatic treatment using polysaccharase and protease. U.S. Pat. 4,904,586 (Japanese Patent Kokai No. 63-287494) teaches the combined use of a polygalacturonase active enzyme and a protease-active enzyme. European Patent Publication No. EP 39962, U.S. patent application Ser. No. 147812/1980 (Japanese Patent Kokai No. 57-5698) proposes the adoption of a composite enzyme having both .beta.-1,3-glucanase- and protease-activities. None of the proposed treatments with protease have, however, proved satisfactorily effective.
Among other enzymatic processes so far proposed are the utilization of a nuclease-active enzyme according to U.S. Pat. No. 4,729,958 (Japanese Patent Kokai No. 61-146193) and the purification by the action of cellulase according to U.S. Pat. No. 4,416,990 (Japanese Patent Kokai No. 57-91194). These processes also have failed to be adequately effective.
Japanese Patent No. 1318520 (Japanese Patent Kokai No. 60-44919) introduces a treatment with the simultaneous addition of lysozyme, N-acetylmuramyl-L-alanine amidase, and peptidase. These enzymes are known to be able to lyse cell walls, but they have only slight action directly on Gram negative bacteria. Despite long treatment time required, their effect is disappointingly low.
Thus, the known methods for removing water-insoluble microbial residues and culture medium-derived undissolved matter, including the methods relying upon centrifugation or cake filtration or upon enzymatic treatment to make the undissolved matter soluble in water, have not produced a satisfactory xanthan gum or solution thereof with the desired high degree of transparency.